1. Field of the Invention
The instant invention relates to a conformal coating that provides excellent shielding against electromagnetic interference (EMI). More particularly, the instant invention relates to conformal coating comprising an insulating layer and a conducting layer. It is particularly preferred that the conducting layer of the instant invention comprises carbon nanotubes.
2. Description of Background
The demand for electronic assemblies in the automotive, aerospace and various other industries has resulted in the annual production of millions of electronic assemblies by manufacturers in the electronics industry. Often, demand has increased to the point that additional processing equipment and floor space is required to meet the growing demand. To enhance their production efficiencies, electronics manufacturers continuously seek to implement new technologies which can increase output without a corresponding increase in capital, floor space and labor.
As is also well known in the art, electronic assemblies are often required to be capable of withstanding hostile operating environments, such as those commonly found in the automotive and aerospace industries. One practice widely accepted in the electronics industry is the use of a conformal coating which forms a protective barrier layer on the circuit board. Conformal coatings are formulated to protect the electronic assembly from moisture and dirt, as well as make the circuit devices mounted to the circuit board more resistant to vibration.
Specifically, a conformal coating is a thin layer of insulating material with a consistent thickness that closely conforms to the shape and contours of the entire substrate, such as a circuit board. This polymeric covering shields metallic junctions and sensitive components from the deleterious effect of the environment. When properly applied, the coating is a barrier against adverse hazards like dust and dirt, moisture, harsh solvents, high atmospheric humidity, airborne chemical vapors and environmental contaminants. Contamination can compromise a circuit board's operational life. When bare circuits are exposed to humid air, thick films of water molecules can form on their surfaces. Moisture on uncoated circuits can induce metallic growth and corrosion. The thicker the water film, the lower the surface insulation resistance, and the greater the effects on electrical signal transmission. This can result in cross talk, electrical leakage from high-impedance circuits and intermittent transmission, all leading to diminished and often terminated circuit performance. Conformal coatings permit closer circuit traces and decreased line spacing popular with high-component densities and minimizing shorts from bridging. Conformal coatings support components so the solder joints do not carry the entire mass of the component. Many coatings can even improve resistance to abrasion, thermal shock and vibration. These coatings are essential in automotive assemblies, industrial controls domestic appliances, certain consumer products, military and aerospace systems, and medical devices.
Conformal coatings find application in device components of cell phones and computers, particularly integrated circuits, printed wire boards, and printed circuit boards.
Generally, conformal coatings have been composed of polymeric materials of the silicone, acrylic, urethane and epoxy families. These families can be divided into groups based on their particular systems and their curing characteristics. For example, there are two-part material systems which cure upon mixing of the two components, one-part solvent-borne systems such as acrylic and hydrocarbon resins, one-part moisture cure systems, such as urethanes, epoxies and silicones, one-part frozen premixed systems, one-part heat-cured systems, ultraviolet (UV) cured systems, and vacuum deposited materials such as parylene, available through the Union Carbide Corporation.
Other than the vacuum depositing materials, the above coating systems are typically applied by dipping, spraying or brushing techniques, and occasionally are deposited as multiple layers. The product design, the coating process and the process capacity will generally dictate which type of coating system can-be applied for a given application.
In recent years a need has developed, particularly in the aerospace industry and in military applications, for electrical connectors providing effective shielding against electromagnetic interference (EMI) and, in certain applications, having the ability to withstand severe EMI conditions.
It is widely understood that highly conducting materials provide EMI shielding. The development of intrinsically conductive organic polymers and plastics has been ongoing since the late 1970's. These efforts have yielded conductive materials based on polymers such as polyanaline, polythiophene, polypyrrole, and polyacetylene (See “Electrical Conductivity in Conjugated Polymers.” Conductive Polymers and Plastics in Industrial Applications”, Arthur E. Epstein; “Conductive Polymers.” Ease of Processing Spearheads Commercial Success; Report from Technical Insights; Frost & Sullivan; “From Conductive Polymers to Organic Metals.” Chemical Innovation, Bernhard Wessling).
A significant discovery was that of carbon nanotubes, which are essentially single graphite layers wrapped into tubes, either single walled nanotubes (SWNTs) or double walled (DWNTs) or multi walled (MWNTs) wrapped in several concentric layers (B. I. Yakobson and R. E. Smalley, “Fullerene Nanotubes: C1,000,000 and Beyond”, American Scientist v.85, July–August 1997). Although only first widely reported in 1991 (Phillip Ball, “Through the Nanotube”, New Scientist, 6 Jul. 1996, p. 28–31), carbon nanotubes are now readily synthesized in gram quantities in the laboratories all over the world, and are also being offered commercially. The tubes have good intrinsic electrical conductivity and have been used in conductive materials.
Heretofore, conformal coating comprising carbon nanotubes that provides EMI shielding has not been disclosed.